The present invention relates to a controller for fluid operated pressure devices, and more particularly, to a controller having an improved spool-sleeve valve configuration.
Although the present invention is equally adapted to any controller for fluid pressure operated devices wherein the controller utilizes a spool-sleeve valve arrangement, it is especially advantageous when used in controllers for power steering systems of the type employed in off-the-road vehicles, and will be described in connection therewith. Furthermore, although the invention will be described in connection with a rotatable spool-sleeve valve arrangement, it should be appreciated that the invention may also be utilized in spool-sleeve valve arrangements which operate in response to relative axial movement.
A controller for a power steering system of the type to which the present invention pertains is described in U.S. Reissue Patent No. 25,126, assigned to the assignee of the present invention. Controllers of the type disclosed in the cited reissue patent have become well known in the art and generally comprise a housing having an inlet and an outlet and a pair of control fluid ports, feeding a power steering cylinder. The vehicle steering wheel is directly connected to the controller and when in the neutral (non-rotating) position, fluid may pass from the inlet through the valve to the outlet (open-center system), or fluid from the inlet may be blocked from passing through the valve (closed center system).
When the steering wheel is rotated in one direction from the neutral position, the valve is displaced and fluid flows from the inlet through the valve, to the meter, then to one of the control fluid ports to move the power steering cylinder. When the steering wheel is rotated in the opposite direction, the valve rotates in the opposite direction and fluid flows from the inlet port through the valve, then through the fluid meter in the opposite direction, then to the other of the control fluid ports to move the power steering cylinder in the opposite direction.
One of the problems associated with the conventional power steering systems and the controllers used therein is steerine wheel "precession", i.e., the position of the steering wheel corresponding to the neutral position of the controller "precesses" or moves slowly in one direction or the other during operation of the system. it is believed that this is caused primarily by imbalance in the fluid flow paths, i.e., the fluid is subjected to a longer path and/or more flow restriction for one direction of steering than for the other. Among the other problems associated with systems and controllers of the type described is internal leakage, primarily between "metered" fluid and "return" fluid. As used herein, the term "metered" fluid refers to a fluid which has been measured by the fluid meter and is then fed to the power steering cylinder. The term "return" fluid refers simply to fluid displaced by the movement of the power steering cylinder which returns to the valve and passes to the outlet port (or tank port). While the above definitions are strictly true only for a controller in which the flow order is: inlet port -- fluid meter -- cylinder -- outlet port, it will be apparent that the invention is equally adapted for controllers having other flow orders, including but not limited to: inlet port -- cylinder -- fluid meter -- outlet port. In connection with the definitions of "metered" and "return" fluids for any particular controller, it should be noted that the remainder of the fluid passing through the spool-sleeve valve is at approximately the same pressure as the "metered" fluid, but has not been metered and hence, may be referred to as "high pressure, non-metered" fluid.